Add wxUSE_WINSOCK2 wxMSW option to include winsock2.h

Including <winsock.h> and <winsock2.h> is incompatible and if the
application wants to use the latter, it may be convenient to define
wxUSE_WINSOCK2 when building wxWidgets instead of having to work around
winsock.h implicit inclusion from include/wx/msw/wrapwin.h.

Closes https://github.com/wxWidgets/wxWidgets/pull/1122

build/cmake/options.cmake

@@ -174,6 +174,7 @@ if(WIN32)
     endif()
     wx_option(wxUSE_DBGHELP "use dbghelp.dll API" ${wxUSE_DBGHELP_DEFAULT})
     wx_option(wxUSE_INICONF "use wxIniConfig")
+    wx_option(wxUSE_WINSOCK2 "include <winsock2.h> rather than <winsock.h>" OFF)
     wx_option(wxUSE_REGKEY "use wxRegKey class")
 endif()